This invention relates generally to aeration tanks for activated-sludge process sewage treatment and is intended to provide a new and improved aeration tank of the type described which is particularly suitable for treating high-BOD (biochemical oxygen demand) domestic sewage or industrial waste to reduce the BOD of such liquid waste to a large extent.
The present invention proceeds from the realization that, with an eye to the advantageous features of the activated-sludge process, best economy and performance are to be obtained with a pressurized aeration tank having an axial-flow pump incorporated therein provided that such tank is operated under specified conditions. As is well known for removal of high BOD levels, a pressurized aeration tank can conveniently be resorted to, which basically utilizes the fact that the amount of air dissolvable increases with rise of the pressure applied, as Henry's law states. With mere use of a pressurized aeration tank, however, any highly efficient BOD removal cannot be realized and it is prerequisite for high-efficiency sewage treatment that the three elements, including BOD, oxygen and aerobic microorganisms, are all brought into contact and mixed with each other rapidly in a uniform fashion. To meet this requirement, the tank should be operated basically upon the principle of vigorous mechanical agitation. However, mere introduction of any vigorous mechanical agitation, which necessitates a large power supply, does not contribute to the BOD removal so much as expected, nor enables any substantial reduction in floor space required for tank installation. Under these circumstances, it is to be noted that the vigorous mechanical agitation can be made the best of only with use of an axial-flow impeller pump.
Description will next be made of the construction and operation of the present invention with reference to the accompanying drawings, which illustrate a preferred embodiment of the invention.